1. Field of the Invention
This invention relates to concrete foundations, and more particularly, to reinforced concrete pier foundations useful for the support of tall, heavy and/or large towers which may be used to support power lines, communication systems, street lighting and signals, bridge supports, commercial signs, freeway signs, ski lifts and the like, and especially wind turbines which are subject to very high upset forces.
2. Background of the Invention
Various different forms of foundations utilizing some of the general structural and operational features of the instant invention have been known. However, these previously known foundations do not include some of the basic features of the instant invention. The combination of features incorporated in the disclosed embodiments of the instant invention enable a heavy duty foundation to be formed in situ or be precast and transported to the construction site, and be placed in position in a manner not requiring the use of large drilling rigs or pile drivers. The present invention is particularly useful in locations where the ground water is high and/or the depth clearance for placement of the foundation is low. The foundation of the present invention is capable of resisting very high upset loads and in a manner independent of the concrete of the foundation experiencing alternating localized compression and tension loading.
In a conventional concrete pier foundation the concrete bears the compressive loads and the contained reinforcing bars (rebar) are sized to bear the tensile loads. Anchor bolts are typically utilized solely for the purpose of attaching the supported structure to the foundation. The anchor bolts are typically placed within a reinforcing steel (rebar) matrix or cage using a removable template at the top and a separate anchor plate at the bottom of each bolt to prevent anchor bolt pullout. The entire module is poured in concrete. Conventional foundations typically resist overturning movement by their weight.
Further, when the foundation is loaded by the structure supported thereon, the foundation is subjected to varying tensile and compressive loads with there being a boundary at the bolt anchor plates where the loading on the concrete alternates from a compressive load to a tensile load depending upon the various forces on the supported structure. The tensile load from the overturning moment of the supported structure is applied near the top of the foundation by the anchor bolts and subjects the large portion of the foundation below the point of application to tension. The large foundation typically requires a great amount of reinforcing steel and a large amount of concrete to encase the reinforcing steel. Extensive labor is also necessary to assemble the reinforcing steel matrix and fill the volume of the foundation with concrete and fix the anchor bolts. A typical cylindrical foundation also requires the use of a large drill to excavate the hole.
My two prior U.S. patents, U.S. Pat. Nos. 5,586,417 and 5,826,387, hereby incorporated by reference in their entirety as if fully set forth, are hereinafter sometimes referred to as “my prior patents”. My prior patents disclose concrete pier foundations under high unit compression which eliminate the necessity for large quantities of reinforcing steel bars (rebar or tension bars) and substantially reduce the amount of concrete necessary. Therefore, the cost of these foundations is substantially less when compared to conventional designs. Furthermore, the foundations of my prior patents simplify the placement of the supported structure on the foundation and eliminate alternating cyclical compression and tension loading on the foundation, thereby substantially reducing fatigue. Also, these patented foundations allow for the replacement of the tower anchor bolts in the unlikely event of bolt failure and allow for removal of the upper four to five feet of the foundation in the event such action is desired for decommissioning purposes.
The foundations of my prior patents are preferably in the shape of a concrete cylinder. The outer boundary shell of the concrete may be formed by corrugated metal pipe (CMP). The inner boundary, preferably in large hollow cylinder foundations, also may be formed by CMP of a lesser diameter. Elongated high strength steel bolts are positioned around the periphery of the concrete cylinder and extend from a peripheral anchor plate or ring near the bottom of the cylinder vertically up through the concrete portion of the foundation to a peripheral connecting plate or flange adjacent the upper end of the structure. The bolts are received in “hollow tubes” to isolate the bolts from the concrete. The steel bolts preferably serve as anchor bolts for attachment of the base flange of the supported tower structure, and the anchor bolt pattern may thus be determined by the bolt hole pattern on the lower base plate or flange of the supported tower. The steel bolt pattern may also be engineered into the construction of the foundation by a removable template.
The “hollow tubes” are preferably elongated plastic tubes or sleeves which encase the bolts substantially through the entire vertical extent of the concrete and prevent the bolts from adhering to the concrete. Then, the bolts can be tensioned after the concrete has hardened and cured in order to post-tension the entire concrete foundation. Alternatively, the elongated bolts can be wrapped in plastic tape, or otherwise sheathed to prevent the bolts from adhering to the concrete during curing and allow the bolts to stretch under tension over the entire operating length of the bolt through substantially the full vertical extent of the concrete. There is typically no conventional rebar reinforcing steel in the foundation, except perhaps in large foundations where a small amount of incidental rebar may be used to stabilize the bolts during construction of the foundation.
The center of a large hollow cylindrical foundation according to my prior patents is filled with excavated soil and then capped. Excavation for the foundation may be done using widely available, fast, low cost excavating machines instead of relatively rare, slow, costly drills necessary for conventional cylindrical foundations.
The foundations of my prior patents use the mechanical interaction with the earth to prevent overturning instead of the mass of the foundation typically used by other foundations for turbine towers and other large structures subject to high upset forces. The high strength anchor bolts are tightened to provide heavy tension on the foundation from a heavy top flange through which the bolts pass to the anchor flange or plate adjacent the bottom of the foundation, thereby post-stressing the concrete in great compression and placing the entire foundation, between the heavy top plate or flange and lower anchor plate or flange, under high unit compression loading. The bolts are tightened so as to exceed the maximum internal tension expected from overturning forces of the tower structure on the foundation. Therefore, the entire foundation withstands the various loads with the concrete thereof always remaining in compression and the bolts always in static tension. The concrete pier foundations of my prior patents may be formed in situ in a remote location or be formed in a pre-cast concrete section placed under heavy compression.
To describe a portion of the foundation structure incorporated in the foundation of the present invention, reference is now made to FIGS. 1, 2 and 2A which are taken from my prior patents. In these Figures, the numeral 10 generally designates the pier foundation. The foundation 10 includes inner and outer upstanding corrugated metal pipe (CMP) sections 12 and 14. The outer section 14 is initially placed within a hole or excavation 16 formed in the ground 18 and rests upon the bottom of the excavation 16. The inner section 12 is then placed and positioned within the excavation 16. The interior of the inner CMP 12 is partially back filled and the excavation 16 outwardly of the outer CMP 14 is partially back filled to stabilize the CMP sections generally in position within the excavation and relative to each other.
The foundation 10 additionally includes a series of tensioning bolts 20 and 21 spaced circumferentially about the annulus defined between sections 12 and 14. The tensioning bolts are preferably in side-by-side pairs which are spaced radially from the center of the foundation. In a typical embodiment, the inner ring of bolts 20 has a slightly shorter diameter than the outer ring of bolts 21. Forty-eight tensioning bolts 20 and forty-eight tensioning bolts 21, or a total of ninety-six, are provided. The rings of bolts have diameters which are several inches apart and which generally equal about 12 feet.
The lower ends of the bolts 20 and 21 are anchored to a lower annular plate or anchor ring 22, sometimes also called an embedment ring, which preferably may be constructed of several circumferentially butted and joined sections. The anchor or embedment ring 22 is radially spaced relative to the inner CMP section 12 preferably by utilizing circumferentially spaced horizontal and radially extending positioning bolts 24 threaded through nuts 26 secured to the under side of the anchor ring 22 at points spaced circumferentially thereabout. Further, the bolts 20 and 21 have all but their opposite ends slidingly received through hollow tubes, such as PVC pipes 30, which are sized to receive and loosely grip bolts 20 and 21 but still permit free movement therethrough. The hollow tubes or PVC tubing 30 need not extend through the entire vertical height of concrete 68, but only through as much of the central portions and extending as close to the top and bottom as to allow the tensioning bolts to extend evenly through the concrete during post-tensioning.
The PVC pipes 30 and other suitable tubing or isolating mechanism serve to allow bolts 20 and 21 to move relatively freely through the concrete after curing so as to allow post-tensioning of the elongate bolts 20 and 21. In addition, rebar wraps 28 are used and secured to the tubes 30 associated with outer bolts 21 at approximately five foot intervals along the vertical extent of the bolts 21 in order to maintain the bolts longitudinally straight during the pour of concrete.
The upper ends of the bolts 20 are supported from a template 32 which consists of upper and lower rings 34 and 36 or ring sections secured together. Upwardly opening radial channel members 38 and mounting blocks 40 received in the channel members 38 are clamped between the upper and lower rings 34 and 36 through the utilization of upper and lower nuts 42 and 44 threaded on the bolts 20 and 21. The inner ends of the radial channel members 38 are joined by a center circular plate 46. The inner portions of the radial channel members 38 include lateral stabilizers 45 in the form of inverted channel members downwardly embracingly engaged thereover and equipped with opposite side set screws 47 clamp engaged with the corresponding channel members 38. The depending flanges of the channel members 45 are slotted for stabilizing engagement with adjacent upper edge portions of the inner pipe 12 while the outer ends of the channel members 38 include threadingly adjustable channel member feet 50 abutingly engageable with the ground 18. A cylindrical form plate 52 is clamped about the upper end of the outer pipe 14 and has its opposite ends secured together in overlapped relation.
Thereafter, concrete 68 may be poured to the bottom of each of the radial channel members 38 and to the top of each of the blockout bodies 64. After the concrete 68 has hardened, the upper nuts 42 are removed and the entire template 32 including the upper and lower rings 34 and 36, the channel members 38 and attached feet 50 are lifted up from the bolts 20 and 21 and the form plate 52, the blockout bodies 64 being exposed from above by removal of the template 32 to thus allow removal of the blockout bodies 64.
When the concrete 68 has sufficiently hardened, it must be determined that the groove 70 is level. If groove 70 is not level, a coating of high compression hardenable grout should be placed within the groove 70 to achieve a levelness for the tower base. Further, even if groove 70 is level, it may be desirable to place grout in the groove 70 as illustrated in FIG. 8 of my U.S. Pat. No. 5,586,417. The nuts 44 are removed or threaded downwardly on the bolts 20 and 21 at least ¾ inch. A tower (not shown) to be supported from the foundation 10 is thereafter lowered into position with the upper exposed ends of the bolts 20 and 21 upwardly received through suitable bores formed in the inner and outer peripheries of the heavy annular plate or base flange of the tower and the lower lug defining portion of the base flange seated in the groove or grout trough 70. Initially the upper nuts 42 are again threaded down onto the upper ends of the bolts 20 and 21 and preferably torqued to about 50 foot pounds. The nuts 42 are thereafter sequentially torqued (in a predetermined pattern of tightening) preferably to about 600 foot pounds which places each of the bolts 20 and 21 under approximately 40,000 pounds tension at approximately ⅓ the stretch limit of the bolts 20 and 21.
As illustrated in FIG. 1 and as described in my U.S. Pat. No. 5,586,417, column 8, in lines 37–43, the backfill within the inner pipe 12 may be completed to substantially ground level and provided with a poured concrete cap 86. The cap 86 may be sloped toward the center thereof and provided with a draining conduit 88, and a conduit 90 for electrical conductors (not shown) also may be incorporated in the foundation 10.
By placing the bolts 20 and 21 under high tension, the cylindrical structure comprising the concrete 68 is placed under heavy unit compressive loading from the upper end thereof downwardly to a level adjacent the lower end of the cylindrical structure, and the unit compressive loading is considerably greater than any upset tensional forces which must be overcome to prevent upset of the tower and foundation 10. As a result, the concrete foundation 68 is always under compression and never subject to alternating compression and tension forces.